1. Technical Field
The present invention generally relates to aqueous ionic absorbent solutions for carbon capture processes. The present invention further generally relates to aqueous ionic absorbent solutions containing an amine functionalized ionic absorbent dissolved in a sufficient amount of diluent for use in a carbon dioxide capture process.
2. Description of the Related Art
The removal of carbon dioxide from the natural gas is commercially practiced now in order to obtain natural gas which satisfies sales specifications or other process-dependent requirements.
Carbon dioxide is one of the primary combustion products as fuel is burned and is emitted into the atmosphere as a waste flow in the flue gas. Removal of carbon dioxide from the flue gas is not commonly practiced at present time. As the efforts to control the CO2 emissions to the atmosphere increases, the removal of carbon dioxide from the flue gas may become necessary and to be practiced in an industrial scale in order to satisfy the carbon dioxide emission requirements which are set by air pollution control authorities. The CO2 removal process from natural gas may not be directly applied to the CO2 removal from flue gas since the conditions of these two processes are very different.
Several processes for removing carbon dioxide from gases are known. Examples of such processes for carbon dioxide separation and capture include chemical absorption, physical and chemical adsorption, low-temperature distillation, gas-separation membranes, mineralization/biomineralization, and vegetation. The carbon dioxide absorption process is a unit operation where one or more components in a gas mixture are dissolved in a liquid (solvent). The absorption may either be a purely physical phenomenon or involve a chemical reaction, such as the reaction between carbon dioxide and an amine. Generally, the liquid solvent is an aqueous amine solution for the removal of carbon dioxide from gas streams.
An example of an absorption process is the process for removing carbon dioxide from flue gas by means of monoethanolamine (MEA) or diethanolamine (DEA). The flue gas is led into an absorption column where it comes into contact with MEA or DEA which absorbs the carbon dioxide molecules. Typically, these amines, MEA and DEA, are used as 25 to 30 wt. % amine in an aqueous solution. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced at the bottom. The solvent is then led to a desorption process where the liquid is heated, and the carbon dioxide molecules are removed from the solvent by means of a desorption column. carbon dioxide and water emerge from the amine solution and the water is separated by condensing the water vapor in a heat exchanger. The solvent is cooled and then recycled back to the absorption tower for additional carbon dioxide absorption.
Solvent chemistry, corrosion, and viscosity consideration limit the amine strength to about 30 wt. % MEA. At flue-gas carbon dioxide partial pressures (e.g., 0.04 to 0.15 atm), the carbon dioxide-rich (“rich”) solvent loading is about 0.42 to 0.45 mol CO2/mol MEA and the CO2-lean (“lean”) solvent loading is about 0.15 to 0.17 mol CO2/mol MEA. The difference in loading (0.25 to 0.3 mol CO2/mol MEA) sets the circulation rate of the amine and influences capital and operating costs.
MEA also has disadvantages in that it has several mechanisms of loss, and a continuous makeup of MEA is required by post-combustion processes. For example, MEA degrades in the presence of oxygen from the flue gas. Thus, to limit the oxidative degradation, corrosion inhibitors may be used. MEA also degrades into heat-stable salts (HSS) from reaction with carbon dioxide. To solve this problem, a reclaimer would be added on the regenerator to separate the HSS from the amine solution to provide suitable makeup MEA. Lastly, the volatility of MEA results in the treated flue gas to contain in excess of 500 ppmv MEA when leaving the absorber to the vent. To address this, a wash section is added at the top of the absorber and makeup water is added to scrub the MEA from the treated flue gas. The mixture is then sent down the column along with the remaining lean solvent to absorb carbon dioxide from the incoming flue gas. Water washing can cut the MEA emissions to about 3 ppmv.
MEA may also degrade over time thermally, thereby limiting the temperature of operation in the absorber and regenerator. With a cooled flue gas inlet temperature of about 56° C., the absorber column may operate at a bottoms temperature of 54° C. and a pressure of 1.1 bar while the regenerator may operate at a bottoms temperature of 1.9 bar and 121° C. (2 bar saturated steam). For 30 wt. % MEA, the amine reboiler steam temperature is kept at less than 150° C. (4.7 bar saturated steam) to limit thermal degradation.
MEA also degrades in the presence of high levels of NOx and SOx which are common in facilities that burn coal and fuel oil. However, if carbon dioxide removal from a high NOx and SOx containing flue gas is desired, separate process facilities such as SCR (Selective Catalytic Reduction) and FGD (Flue Gas Desulfurization) are needed for removal of NOx and SOx, respectively.
Accordingly, to have a superior, post-combustion carbon dioxide removal technology that is better than those known in the art (30 wt. % MEA and similar aqueous amines), it is desirable to develop an improved carbon dioxide absorption solvent and a process for its use.